M. Guainazzi
XMM-Newton Science Operation Center, VILSPA, ESA, Apartado 50727, 28080 Madrid, Spain
Received 26 August 2002 / Accepted 30 January 2003
Abstract
This paper presents ASCA (July 1997), XMM-Newton (December 2000) and BeppoSAX
(January 2001) observations of the Piccinotti Seyfert 1 galaxy
ESO 198-G24.
The BeppoSAX 0.1-200 keV spectrum exhibits reprocessing features,
probably produced by an X-ray illuminated,
relativistic accretion disk subtending a solid angle
.
During the XMM-Newton observation
the fluorescent iron
line
profile (centroid energy
keV)
was broad and twice as bright as in the BeppoSAX observation.
An additional emission feature (
keV),
detected at the 96.3% confidence level, may be part
of a relativistic, double-peaked profile.
By contrast, in the earlier ASCA observation
the line profile is
dominated by a remarkably narrow "core''
(intrinsic width,
eV). If
this component is produced by
reflection off the inner surface of a molecular torus, its
large Equivalent Width (
300 eV) most likely represents
the "echo'' of a previously brighter flux state,
in agreement with the dynamical
range covered by the historical
X-ray light curve in ESO 198-G24.
Key words: accretion, accretion discs - galaxies: active - galaxies: individual: ESO 198-G24 - X-rays: galaxies - galaxies - nuclei
The discovery of the broadened and skewed
fluorescent iron
line profile
in the first "long-look''
ASCA observation of MCG-6-30-15 (Tanaka et al. 1995)
fostered the hope that general relativistic effects
could be observationally studied through X-ray
spectroscopy of nearby Active Galactic Nuclei (AGN).
This hope was
further strengthened by the discovery that about 50% of
the known bright Seyfert 1s exhibit broad line
profiles (Nandra et al. 1997). It was reasonable to expect that
the advent of Chandra and XMM-Newton would
allow to significantly extend the depth and range
of these studies.
These hopes are now facing a complex reality. In a few objects the existence of a relativistically broadened iron line profile is out of question (Nandra et al. 1999; Wilms et al. 2001; Turner et al. 2002; Fabian et al. 2002). However, often the observed profiles do not match the theoretical calculations (Fabian et al. 1989; Laor 1991; Matt et al. 1992). Electron scattering, as often observed in type 2 Seyferts (Ueno et al. 1994; Turner et al. 1997; Matt et al. 2000), may help reconciling the difference between observed and theoretical line profiles produced in X-ray illuminated, relativistic accretion disks (Reeves et al. 2001; Matt et al. 2001). Other pieces of evidence suggest constraints on the nature of the accretion: the disk may be truncated at a radius where relativistically broadened wings are negligible (see, e.g. the discussion in O'Brien et al. 2001), or may develop a hot "skin'' (Nayakshin et al. 2000; Ballantyne et al. 2001), which may contribute to the iron line profile through transitions of highly ionized stages. Observations with the Chandra high-energy gratings (Yaqoob et al. 2001; Kaspi et al. 2002; Weaver 2001, and references therein) and with XMM-Newton (Gondoin et al. 2001a,b; Pounds et al. 2001; Petrucci et al. 2002) have often unveiled narrow line components. When measured at the currently highest possible energy resolution, their intrinsic velocities are consistent with the width of the optical broad lines, suggesting an origin in the same medium. However, different origins (e.g. scattering off the inner side of the molecular "torus'' envisaged by the Seyfert unification scenarios; Antonucci & Miller 1985; Antonucci 1993) cannot be ruled out. These narrow components have Equivalent Widths (EW) typically as large as 100 eV, and may contribute as much as 50% to the total iron line flux (Weaver 2001).
In this context, we present in this
paper X-ray observations
of the Seyfert 1 galaxy ESO 198-G24 (
z = 0.0455).
Relatively little is known on its X-ray
spectral properties, despite the fact that it
belongs to the
Piccinotti sample (H0235-52;
Piccinotti et al. 1982), and hence it is one of
the brightest AGN of the 2-10 keV
sky. In their EXOSAT AGN survey paper, Turner & Pounds
(1989) note that it exhibits a "canonical spectrum''
(photon index,
)
with "no significant low-energy absorption'', and
a 2-10 keV luminosity
erg s-1, about three times lower than
during the HEAO-1 scans. In the ROSAT
All-Sky Survey (Schartel et al. 1997) a rather steeper
soft spectral index was found (
),
again without any evidence for absorption. The
0.1-2.4 keV observed flux
[
erg cm-2 s-1]
corresponds to a luminosity of
erg s-1once corrected for Galactic absorption
(
cm-2;
Dickey & Lockman 1990) and extrapolated into the 2-10 keV band.
Malizia et al. (1999) report a detection by
BATSE, with a 2-100 keV flux of
erg cm-2 s-1.
To our knowledge, no result has ever been published from
ASCA, BeppoSAX or XMM-Newton observations of this source so far.
This paper attempts to address
this lack in the literature.
The log of the observations presented in this paper
is reported
in Table 1, together with the
corresponding exposure times and count rates
Mission | Observation date | Exposure time | Count rates |
(ks) | (s-1) | ||
ASCA (SIS0/GIS2) | July 10, 1997 | 31.1/34.3 |
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XMM-Newton (pn/RGS1) | December 1, 2000 | 6.8/13.0 |
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BeppoSAX (LECS/MECS/PDS) | January 23, 2001 | 55.1/143.3/51.3 |
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In this paper only the time-averaged
pn (Strüder et al. 2001) spectrum in the 2-15 keV band will be
presented.
No significant spectral variability
is associated with a 7%
flux fluctuation observed during the
XMM-Newton observation.
The source was outside the MOS1 field of view.
The MOS2 exposure was performed
in Timing Mode. The calibration of
this mode is still preliminary, and the corresponding data
will not de discussed in this paper.
The pn
observation was performed in Small Window Mode with the
blocking optical
Medium filter. Data were reduced with SAS V5.3.3
(Jansen et al. 2001),
using the most updated calibration files
available as of July 1, 2002. A spectrum including
single- and double-pixel events was extracted. It was
verified that
spectra extracted with either only single- or only double-pixel
events are mutually consistent within the statistical uncertainties.
Source spectra were extracted from a circular region of
1.8
radius.
Non X-ray background remained low throughout the observation,
hence the
whole integration time of the observation was used.
Several recipes to extract
the background spectra were compared,
and yielded very similar
results.
The results presented in this paper
were obtained with
background spectra
extracted from
blank field templates available at the XMM-Newton
Science Operation Center (Lumb et al. 2002), and rescaled
in order to match the observed
spectrum in the 15-20 keV energy band.
The background contributes around 3% and 8% of the total spectrum at 2 and 5.5 keV respectively,
and is brighter than the source above 10 keV. The spectra
used
throughout this paper were rebinned
in such a way that: a) the intrinsic
energy resolution of the detectors is sampled
by a number of spectral channels not larger than 3; b) each spectral bin has at least 50 counts,
to ensure the applicability
of the
test. The same criteria
were applied for the BeppoSAX and ASCA spectra discussed
later, but the number of counts in these cases was
limited to 25.
All the models discussed in this Section and
in the following ones are
modified by photoelectric absorption, whose
column density, ,
is
constrained to be not lower than the contribution
due to the interstellar matter in our Galaxy along the line-of-sight
to ESO 198-G24.
A simple power-law model is a marginally acceptable
description of the pn spectrum
(
degrees of freedom, d.o.f.).
A count excess around 6 keV (observer's frame) is present. The
addition of a narrow (i.e. intrinsic width,
,
equal to 0)
Gaussian emission profile to the best-fit power-law yields
an improvement of the
by 18.3
for a decrease by 2 in the number of degrees of
freedom (this quantity will be indicated
as
hereinafter), significant at the
99.992% confidence level. Leaving the intrinsic width
of the Gaussian profile free in the
fit yields a further improvement in its
quality (
,
significant at the 98.8% confidence level).
The best-fit parameters of the line are:
eV,
centroid energy
keV, and
eV.
The centroid
energy is consistent with
fluorescence from neutral or mildly
ionized iron, and strictly inconsistent with
species more ionized than Fe XIX.
The intrinsic width is constrained to be larger than
about 40 eV at the 90% confidence level for two
interesting parameters (see Fig. 1). The upper
limit is more loosely constrained, and widths as large as
0.5 keV are in principle possible.
The profile of the iron line may be more complex
than a single broad Gaussian.
A careful inspection of the residuals (see Fig. 2)
suggests an additional emission
feature with
keV
(observer's frame).
If an additional Gaussian profile
is added to the model, the
quality of the fit
is improved at the 96.3% confidence level
(
).
The best-fit values of this component are
keV,
eV and
eV.
In order to compare its significance with the
current systematic uncertainties of the pn response matrix, we
analyzed pn public data of featureless calibration sources,
acquired in Small Window Mode and
reduced under the same conditions as the
ESO 198-G24 data. Typical systematic
uncertainties (represented by the shaded area in
Fig. 3) are
5% in the 5-7 keV
band. We rule out therefore an instrumental origin
for the
keV feature.
Table 2 summarizes the spectral results.
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Figure 1:
Iso-![]() ![]() ![]() ![]() |
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Figure 2: Spectrum ( upper panel) and residuals in units of standard deviations ( lower panel) when the best-fit model as in Table 2 is applied to the pn data of ESO 198-G24. In the inset, the data/model ratio in the 5-7 keV energy range is shown, when the best-fit continuum is determined after the data points in the 5.0-6.5 keV (observer's frame) are removed. |
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Figure 3:
Ratio of the ESO 198-G24 pn spectrum
against the best-fit continuum model
as in Table 2
( filled circles). The
shaded area represents the ![]() ![]() |
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BeppoSAX | XMM-Newton | ASCA | |
Continuum | |||
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<0.5 | ![]() |
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"6.4 keV'' feature | |||
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I![]() |
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EW (keV) |
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"5.7 keV'' feature | |||
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... |
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... |
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... |
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... |
I![]() |
... |
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... |
EW (keV) | ... | ![]() |
... |
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1.14 | 1.11 | 1.06 |
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... | 1.06 | ... |
a 2-10 keV observed flux in units of
10-12 erg cm-2 s-1.
b Unabsorbed 0.1-100 keV luminosity in units of 1044 erg s-1.
c Column density of the
intrinsic (i.e. at
d In units of 10-5 photons cm-2 s-1. |
The
width of the
keV feature is not
due to the presence of the additional line
at
keV. If the data are
fit with
a model constituted by a power-law
and one Gaussian profile,
after removing the data points in the
5.0-5.5 keV energy range (observer's
frame), the parameters of the Gaussian
profile are:
keV,
eV,
eV,
therefore indistinguishable from those
obtained with the complete model on
the whole 2-15 keV energy band.
No further lines are statistically required by the fit. No evidence exists for photoelectric absorption edges in the pn spectrum either. The 90% upper limits on the optical depth of un-blurred Fe I, Fe XXV and Fe XXVI K edges are 0.14, 0.24 and 0.30, respectively.
ASCA data were retrieved from the public HEASARC archive as
screened event lists. Data reduction followed standard
procedures as described in, e.g., Guainazzi et al. (2000).
Spectra of all the ASCA instruments were integrated on the
whole elapsed time
of the observation, after verifying that one can neglect
spectral variability effects, and were
simultaneously
fit, allowing free normalization factors for
the individual instruments to
account for systematic uncertainties in the
cross-normalizations (7%). A simple power-law
continuum modified by intervening photoelectric absorption
yields a marginally acceptable
fit to the data (
d.o.f.).
The 2-10 keV flux is about two times weaker than in the
XMM-Newton observation.
The residuals exhibit a clear narrow excess
feature at about 6 keV (observer's frame). The addition of a narrow
Gaussian profile largely improves the quality
of the fit (
,
significant at the 99.9998% confidence level).
The centroid energy is again consistent with neutral or
mildly ionized iron (
).
No additional improvement in the
is obtained
if the line width is left free in the fit. The 90% upper limit on its
is 50 eV only. Similarly, the inclusion of a
further Gaussian emission profile with
keV
is not statistically required
(
).
A summary of the best-fit results is shown in Table 2. The ASCA spectra and corresponding best-fit model are shown in Fig. 4.
BeppoSAX data were retrieved from the
A.S.I. BeppoSAX Science Data Center (ASDC)
public archive. Spectra
from the Low Energy Concentrator Spectrometer (LECS,
0.1-4 keV; Parmar et al. 1997) and the Medium Energy
Concentrator Spectrometer (MECS, 1.8-10 keV;
Boella et al. 1997) were extracted from linearized
and calibrated event lists, following standard recipes
as detailed in, e.g., Guainazzi et al. (1999).
Background spectra were extracted from blank
sky field event lists provided by the
ASDC, using the same area in detector coordinates
as the source.
Extraction
radius were 8
and 4
for the LECS and the MECS, respectively.
Background-subtracted spectra for the Phoswitch Detector System
(PDS, 13-200 keV; Frontera et al. 1997) were generated by
plain subtraction of the 96 s duty-cycle intervals, when
the collimators were pointing to the line of sight towards
ESO 198-G24 and to a region 3.5
degrees aside.
Spectra were integrated on the whole elapsed time
of the observation, after verifying that one can neglect
spectral variability effects,
and were simultaneously fit after applying correction
factors, to account for known differences in the absolute flux
cross-calibration of the instruments
(in particular, a value of 0.84
was employed for the PDS versus MECS normalization;
Fiore et al. 1998).
A marginally acceptable fit
(
d.o.f.) is obtained
with the standard Seyfert 1 continuum model in the BeppoSAX
energy bandpass (Perola et al. 2002),
i.e., a photoelectrically absorbed
power-law, modified by Compton-reflection from cold
matter (Lightman & White 1988; George & Fabian 1991; Magdziarz & Zdziarski 1995).
Assuming
an inclination angle of the reprocessing matter
,
R
is comprised
between 0.1 and 1.0 at the 90% level for
two interesting parameters (cf. the left panel of
Fig. 5).
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Figure 4: Spectrum ( upper panel) and residuals in units of standard deviations ( lower panel) when the best-fit model as in Table 2 is applied to the ASCA data of ESO 198-G24. |
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Figure 5:
Iso-![]() |
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Any thermal cut-off in the intrinsic power-law is constrained
to lay at energies
eV at
the same confidence level
(cf. the right panel of Fig. 5).
The spectral index of the intrinsic power-law
(
)
is remarkably close
to the XMM-Newton and ASCA observed
indices.
The BeppoSAX best-fit parameters
values are only marginally affected
by different choices of the inclination angle,
e.g.:
R changes by 0.1 and
by 0.02
if
.
Excess residuals around
keV (observer's frame)
again suggest emission from a fluorescent
iron
line. The addition of a Gaussian profile
yields an improvement in the quality of the fit
by
,
significant at
the 93.5% confidence level.
No conclusion can be drawn on the intrinsic width
of the line profile.
Its total EW does not exceed 130 eV. A summary of the
best-fit results is shown in Table 2.
The best-fit
model and residuals are shown in Fig. 6.
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Figure 6: Spectrum ( upper panel) and residuals in units of standard deviations ( lower panel) when the best-fit power-law plus Compton-reflection model is applied to the BeppoSAX data of ESO 198-G24. The line is accounted for by a single broad Gaussian profile. |
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Table 2 summarizes the best-fit
results for the ASCA,
XMM-Newton and BeppoSAX observations of ESO 198-G24,
when the iron line is
fit by a (combination of) Gaussian profile(s). These observations
span a range of about 3 in 2-10 keV X-ray flux, whereas
the spectral shape of the continuum does not exhibit
significant changes. Anecdotically, the ASCA observation
caught ESO 198-G24 in its
faintest state ever reported. Some of the properties of the iron line
vary in a way, which is apparently correlated with the
X-ray flux. The line EW decreases with increasing
continuum flux, from
eV in the
weakest (ASCA)
to the
eV in the brightest (BeppoSAX)
state. The narrow profile
of the 6.4 keV emission line
measured by ASCA (
eV)
is formally inconsistent with the broad profile
measured by XMM-Newton (
eV).
The centroid energy is always consistent with that of
fluorescent transitions of neutral or mildly ionized
iron. However, alternative interpretations are possible,
when the line profile is significantly broadened
or skewed.
Broad and asymmetric iron line profiles in AGN are naturally explained as due to relativistic effects, affecting the photons emitted in an X-ray illuminated accretion disk (Fabian et al. 1989; Laor 1991; Matt et al. 1992). If the line photons are emitted - originally with a monochromatic energy distribution - within a few gravitational radii from the source of an intense gravitational potential, the combination of kinematic (Doppler) and gravitational shifts over a range of disk annuli can produce significant broadening and skewing of the profile.
The iron line profile can be best studied in the pn spectrum,
thanks to its combination of photon statistics and instrumental
energy resolution.
Bearing the above scenario in mind, the observed
excess in the 5-6.5 keV (observer's frame)
energy range was fit with
relativistic emission line profiles, as expected around a Schwarzschild
(model DISKLINE in XSPEC; Fabian et al. 1989) or a Kerr
(model LAOR in XSPEC; Laor 1991) black hole. These
models depend on a number of parameters: apart
from the centroid energy and the normalization,
the inner ()
and outer (
)
radius of the
line emitting region in units of Schwarzschild
radii (
), the inclination of the accretion disk (i),
and the power-law of the radial emissivity dependence (q). The available statistics
is not sufficient to allow all these parameters to be
simultaneously constrained by the fit. The fits were therefore
performed in two steps. In the first
step, all the parameters were left free to
vary, and their confidence intervals calculated to determine
which of them are unconstrained. The unconstrained parameters
were then frozen to physically plausible values and/or
intervals in the second step. The
final results are summarized in Table 3.
Observation | ![]() |
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q | i | EW |
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(keV) | (![]() |
(![]() |
(![]() |
(keV) | |||
Schwarzschild profile | 6.40+0.17 |
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-2a |
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1.08 |
Kerr profile | 6.40 +0.05 |
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-3a | <39 |
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1.09 |
a Unconstrained and therefore fixed in the fit to the reported value. |
The comparison between the best-fit
relativistic profiles and the data is shown in Fig. 7.
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Figure 7:
Spectrum ( crosses) and best-fit
model in the 4.5-7 keV energy
band, when the iron
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Limited constraints can be derived on the
iron line shape from either the ASCA or the
BeppoSAX observation. The ASCA line is clearly narrow
and no hint of a broad component exists.
Nonetheless, the upper limits
on the EW of an underlying relativistic
profile, with the same shape as observed by XMM-Newton,
are not particularly demanding: 720 eV and
300 eV for a Schwarzschild and a Kerr profile,
respectively (cf. Fig. 8).
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Figure 8: Residuals (in data/model ratio) when a purely relativistic profile, with all its parameter (except the normalization) constrained within the best-fit XMM-Newton confidence intervals (cf. Table 3), is fit to the ASCA iron emission line. |
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The history of the iron
line profile in ESO 198-G24 can
be summarized as follows (in order of increasing
2-10 keV flux):
The EW of the line observed by BeppoSAX
a few weeks later is by a factor of 2 weaker
than measured by XMM-Newton, whereas the
continuum flux was about 60% more intense.
If the BeppoSAX profile is dominated
by a broad component, this evidence
suggests that the
response time of the relativistic line
to variations of the underlying continuum is probably
larger than at least a few hours.
If the solid angle subtended by the
reprocessing matter and the spatial distribution
of the intrinsic continuum did not change between
the two observations (which is consistent with
the closeness between the intrinsic
spectral index measured by BeppoSAX and the
observed spectral index measured
by XMM-Newton),
the larger EW of the relativistic
profile in the pn spectrum can be explained by
a delayed response to a brighter continuum flux
state before the start of the pn exposure. The
delay is at least of the order of the pn exposure
elapsed time, i.e. 2 hours,
corresponding to a spatial scale
of
,
if M8 is the black hole
mass in units of 108 solar masses.
Fabian et al. (2002) point out similar
problems to explain the short time scales
variability pattern
of the relativistic line in MCG-6-30-15.
If the BeppoSAX profile is substantially
"contaminated" by a narrow component, the discrepancy
between the broad component EWs is obviously even larger.
By contrast, the ASCA line profile is dominated
by a remarkably narrow "core''.
Unresolved iron lines have
been now quite commonly discovered both in Chandra
grating (Yaqoob et al. 2001; Kaspi et al. 2002)
and in XMM-Newton observations
(Gondoin et al. 2001a,b;
Reeves et al. 2001; Petrucci et al. 2002; O'Brien et al. 2001).
The two most plausible possibilities are
Compton-reflection by matter far off the central engine
(e.g., the "torus''), or
gas clouds in the Broad Line Regions.
The EW of the narrow line in ASCA (300 eV) is
much larger than typically observed
(50-100 eV). If it originates in the torus,
one may expect
eV if the torus subtends
a solid angle
(Krolik et al. 1994;
Ghisellini et al. 1994). Such a large EW may be therefore
indicative
either of a strong (a factor
10) iron
overabundance or, more likely, of the "echo'' of
a brighter illuminating flux state.
Our sparse knowledge of the historical X-ray light
curve of ESO 198-G24 indeed suggests a dynamical
range of at least a factor 6 in the nuclear
power.
Acknowledgements
Comments by an anonymous referee strongly contributed to improve the quality of the presentation and sharpen the focus of this paper. This paper is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). The XMM-Newton Science Archive (XSA) Development Team is gratefully acknowledged for its highly professional work. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center and of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
ESO 198-G24 is one of
the brightest Seyfert 1s where
a fluorescent iron
line and a Compton reflection
continuum have been simultaneously measured.
The amount of reflection (assuming an inclination angle
as derived by the fit of the
iron
line in the pn spectrum with a Schwarzschild profile:
)
is
,
about one-half the value expected if
the reprocessing occurs in a plane-parallel, semi-infinite
slab, and the primary emission is isotropic.
![]() |
Figure 9: 2-10 keV luminosity versus the Compton-reflection relative intensity R for a PDS count rate limited sample of publicly available type 1 ( filled circles) and type 2 ( empty squares) Seyferts observed by BeppoSAX. |
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Nandra et al. (1997b) and Reeves &
Turner (2000) discuss a possible "X-ray Baldwin effect'',
whereby the intensity of the reprocessing features
decreases with increasing AGN X-ray luminosity due to
a higher degree of ionization of the accretion disk,
which smears the spectral contrast between the reflected
and direct continua and shifts the iron line centroid
either to intermediate ionization stages,
where
resonant scattering may cause
a reduction in the line flux (Matt et al. 1993, 1996),
or to fully ionized stages. In ESO 198-G24 there
is no compelling evidence for high iron ionization stages to be
responsible for the bulk of the line profile in any
of the scenarios discussed in this paper.
In order to test the dependence of the
Compton reflection on the luminosity,
we retrieved from the BeppoSAX public archive data
for a sample of Seyfert galaxies (of both type 1 and 2),
having a PDS count rate larger then 0.35 s-1. We
analyzed these observation, applying the standard
Seyfert spectral template, as described in Perola et al. (2002),
eventually modified for intervening photoelectric absorption
in type 2 objects. The reprocessor is assumed always
neutral, and the inclination angle
as in Sect. 3. The R versus 2-10 keV luminosity (LX) plot is shown in Fig. 9.
There is no evidence for any correlations between these
quantities, across the 2.5 dex span in luminosity.